This invention relates to an encoding communication system used with facsimiles or the like for transmitting images of pictures and writings at high speed. Particularly the present invention relates to a code selection device and a mode selection device used in such an encoding communication system.
Since picture elements constituting the total picture or writing area generally have a two-dimensional correlation, any object picture element destined to be next encoded has an information content which differs with the state to which it belongs. The term "state" used herein means the pattern in which picture elements adjacent to that object picture element and selected as reference points are arranged. Therefore, for effectively encoding picture elements it is required to allot to those picture elements codes whose code lengths depend upon their occurrence frequencies. Accordingly, when only one code form is used, the resulting efficiency has a limit. It is desirable to specify the property of the information sources and prepare a desired number of code forms for selecting from among these code form that code form suited for the property of the particular information source.
Also because many pictures and writings require generation of code word-lengths different from one point to another thereon, the data is often temporarily stored in a memory and read out from the memory at a constant speed matching with the speed at which the readout codes are delivered to the transmission channel involved. Under these circumstances, if code words to be transmitted having low information content may be continuously delivered to the channel then the code words delivered to the channel are generally intermittent. For this reason, it has been previously proposed to additionally prepare special dummy codes and deliver them to the channel independently of picture elements, when the content stored in the memory is reduced to a predetermined magnitude or less.
Consequently, the most desirable form of such a system is first to prepare a desired number of code forms and then to normally select that code form having the shortest code length whereas when the content in the associated memory is reduced to a predetermined magnitude or less, selecting the code form having the shortest code length among the remaining code forms as determined by the particular code delivery speed. In such a system it has been necessary to perform the encoding operation concerning all the code forms prepared for coded signal sequences involved in order to effect the code selection, to provide a memory in which codes with different code lengths are stored, and to compare the code lengths with one another. This has resulted in the problem that, as the number of the code forms increases, the processing speed must increase and the memory capacity for the code length must increase and so on. Thus the measure as described above has scarcely been put to practical use and there have been generally employed systems including fixed codes and dummy codes additionally prepared.
Accordingly it is a general object of the present invention to eliminate the disadvantages of the prior art practice as described above.
It is an object of the present invention to provide in an encoding communication systems a new and improved code selection device for counting the binary symbols included in a sequence of signals to be encoded and utilizing the resulting counts without the necessity of performing the encoding operation concerning all available code forms and counting the code lengths.
It is still another object of the present invention to provide in an encoding communication systems a new and improved code selection device for selecting codes suited for the property of the particular information source to thereby eliminate the necessity of increasing the processing speed in accordance with the number of available code forms and also of providing a code-length memory.
It is a further object of the present invention to provide a new and improved encoding communication system eliminating the transmission of a pause-of-mode signal indicating a change from one mode to another mode to increase the efficiency of transmission of coded signals.